Induced Seismicity Related to Hydrothermal Operation of Geothermal Projects in Southern Germany - Observations and Future Directions

Abstract:

Geothermal power plants in Southern Germany are operated hydrothermally and at low injection pressures in a seismically inactive region considered very low seismic hazard. For that reason, permit authorities initially enforced no monitoring requirements on the operating companies. After a series of events perceived by local residents, a scientific monitoring survey was conducted over several years, revealing several hundred induced earthquakes at one project site.
We summarize results from monitoring at this site, including absolute locations in a local 3D velocity model, relocations using double-difference and master-event methods and focal mechanism determinations that show a clear association with fault structures in the reservoir which extend down into the underlying crystalline basement. To better constrain the shear wave velocity models that have a strong influence on hypocentral depth estimates, several different approaches to estimate layered vp/vs models are employed.
Results from these studies have prompted permit authorities to start imposing minimal monitoring requirements. Since in some cases these geothermal projects are only separated by a few kilometers, we investigate the capabilities of an optimized network combining the monitoring resources of six neighboring well doublets in a joint network. Optimization is taking into account the -- on this local scale, urban environment -- highly heterogeneous background noise conditions and the feasibility of potential monitoring sites, removing non-viable sites before the optimization procedure. First results from the actual network realization show good detection capabilities for small microearthquakes despite the minimum instrumentational effort, demonstrating the benefits of good coordination of monitoring efforts.